Exagon Exagon - 1 month ago 18
C++ Question

boost spirit x3 double parser with restrictions

I am currently parsing doubles using boost spirit x3 with this parser:

boost::spirit::x3::real_parser<double, x3::strict_real_policies<double> > const strict_double = {};


but it also parses doubles like
.356
and
356.
I would like to avoid this, and having the user write
0.356
and
356.0
instead.
Hoow can I apply such an restriction on this existing parser?
Is there a way without writing my own double parser from the scratch?

jv_ jv_
Answer

You can very easily create a custom policy that does what you want:

template <typename ValueType>
struct really_strict_real_policies : boost::spirit::x3::strict_real_policies<ValueType>
{
    static bool const allow_leading_dot = false;
    static bool const allow_trailing_dot = false;
};

Full example (Running on WandBox)

#include <iostream>
#include <boost/spirit/home/x3.hpp>

template <typename ValueType>
struct really_strict_real_policies : boost::spirit::x3::strict_real_policies<ValueType>
{
    static bool const allow_leading_dot = false;
    static bool const allow_trailing_dot = false;
};

template <typename Parser>
void parse(const std::string& input, const Parser& parser, bool expected)
{
    std::string::const_iterator iter=input.begin(), end=input.end();

    bool result = boost::spirit::x3::parse(iter,end,parser);

    if( (result && (iter==end))==expected )
        std::cout << "Yay" << std::endl;
}

int main()
{
    boost::spirit::x3::real_parser<double, really_strict_real_policies<double> > const really_strict_double = {};

    parse(".2",really_strict_double,false);
    parse("2.",really_strict_double,false);
    parse("2.2",really_strict_double,true);
}